The use of accelerometers in prior art subterranean surveying techniques for determining the direction of the earth's gravitation field at a particular point is well known. The use of magnetometers or gyroscopes in combination with one or more accelerometers to determine direction is also known. Deployments of such sensor sets are well known to determine borehole characteristics such as inclination, azimuth, positions in space, tool face rotation, magnetic tool face, and magnetic azimuth (i.e., an azimuth value determined from magnetic field measurements). While magnetometers and gyroscopes may provide valuable information to the surveyor, their use in borehole surveying, and in particular measurement while drilling (MWD) applications, tends to be limited by various factors. For example, magnetic interference, such as from magnetic steel or ferric minerals in formations or ore bodies, tends to cause a deflection in the azimuth values obtained from a magnetometer. Motors and stabilizers used in directional drilling applications are typically permanently magnetized during magnetic particle inspection processes, and thus magnetometer readings obtained in proximity to the bottom hole assembly are often unreliable. Gyroscopes are sensitive to high temperature and vibration and thus tend to be difficult to utilize in MWD applications. Gyroscopes also require a relatively long time interval (as compared to accelerometers and magnetometers) to obtain accurate readings. Furthermore, at low angles of inclination (i.e., near vertical), gyroscopes do not provide accurate azimuth values.
U.S. Pat. No. 6,480,119 to McElhinney, hereafter referred to as the '119 patent, discloses “Gravity Azimuth,” a technique for deriving azimuth by comparing measurements from accelerometer sets deployed along, for example, a drill string. The term “gravity azimuth” as used herein refers to the conventional techniques disclosed and claimed in the '119 patent. Using gravity as a primary reference, the '119 patent discloses a method for determining the change in azimuth between accelerometer sets disposed along a drill string, for example. The method assumes a known displacement between the accelerometer sets and makes use of the inherent bending of the bottom hole assembly (BHA) between the accelerometers sets in order to measure the relative change in azimuth.
Moreover, as also disclosed in the '119 patent, derivation of the azimuth conventionally requires a tie-in reference azimuth at the start of a survey section. Using a reference azimuth at the start of a survey results in subsequent surveys having to be referenced to each other in order to determine the well path all the way back to the starting tie-in reference. One conventional way to achieve such “chain referencing” is to survey at depth intervals that match the spacing between two sets of accelerometers. For example, if the spacing between the sets of accelerometers is 30 ft then it is preferable that a well is surveyed at 30 ft intervals. Optimally, though not necessarily, the position of the upper set will overlie the previous lower set.
Surveying in this way is known to be serviceable, however, potentials for improvements have been identified. First, when relating back to a tie-in reference, the survey interval is dictated by the spacing between the sets of accelerometers, possibly causing more surveys and time to be taken than is necessary to survey the borehole and also possibly causing compounding azimuth errors for survey points further down the chain. Second, surveys cannot be taken independently at any position, because they must be related back to the tie-in reference. It would therefore be highly advantageous to enhance gravity based surveying deployments with additional referencing, so that relation back to a tie-in reference might not always be necessary.
The method described and claimed in the '119 patent does not account for any azimuth misalignment (such as a rotational offset) that may be present between the accelerometer sets. Such misalignment, if not connected or accounted for, may introduce significant error to the determined azimuth values. Thus it would also be advantageous to enhance gravity based surveying deployments with an error correction aspect capable of determining and correcting for any azimuthal misalignment between the accelerometer sets.
The method described and claimed in the '119 patent also does not account for the presence of other subterranean structures, such other boreholes, in a surveyed region. For some applications, such as well avoidance and/or well kill applications, it may be desirable to measure the location of other boreholes in relation to the surveyed borehole. Thus it would also be advantageous to enhance gravity based surveying deployments with a passive ranging aspect capable of determining the location of nearby subterranean structures.